When a user listens to music with earphones or headphones, the sound resonates in the space formed by his/her ears and the earphones or the headphones. The resonance phenomenon causes the user to hear an unnatural sound. To avoid such an unnatural sound, there has been proposed a system aimed at canceling the resonance phenomenon in the space formed by the ears and the earphones or the headphones.
For example, Japanese Patent Application Publication (KOKAI) No. 2009-194769 discloses a conventional technology for cancelling a peak of a resonant frequency detected by specific earphones for measurement purposes provided with a microphone. According to the conventional technology, a sound source signal is output from the earphones. While the earphones are placed in the ear canal, the microphone picks up sound to obtain the frequency characteristics of the acoustic signals. The resonant frequency of the ear canal is detected from the frequency characteristics to reduce the resonant frequency.
Japanese Patent Application Publication (KOKAI) No. H9-187093 discloses a conventional technology for determining a specific variable to suppress the resonant frequency. According to the conventional technology, since music listening in high volume may damage hearing ability, the sound level is reduced to around the resonant frequency of the human ear.
The latter conventional technology does not identify the relation between earphones and ears. Although it is described how to reduce the sound level for only a single resonance, there is not always only one resonant frequency band. It is often the case that a plurality of orders of resonant frequency bands are present. These resonant frequency bands to be reduced differ depending on the environment such as individuals or earphones.
With the above conventional technologies, the earphones need to have a specific structure that the microphone is integrated with the earphone player, and the resonant frequency has to be measured by picking up sounds using the microphone. This means that the conventional technologies cannot be realized by commonly used earphones. The microphone of the specific earphones measures the resonant frequency in a space formed by the specific earphones and ears, and the resonant frequency is different from that when the user uses common earphones. That is, the conventional technologies are not applicable to common earphones and cannot reduce the resonant frequency that varies according to a combination of user's ears and earphones. Therefore, sound cannot be reproduced in high quality.